Aluminum Alloy Processing Method and Aluminum Alloy Workpiece

ABSTRACT

Provided is a method for processing art aluminum alloy comprising: 0.5% by mass or more and 1.0% by mass or less of Mg, 0.5% by mass or more and 3.0% by mass or less of Si, 0.2% by mass or more and 0.4% by mass or less of Cu, 0.15% by mass or more and 0.25% by mass or less of Mn, 0.1% by mass or more and 0.2% by mass or less of Ti, and 120 ppm by mass or less of Sr, the method comprising casting the aluminum alloy and forging the cast aluminum at; a temperature of 200° C. or more and 470° C. or less.

This application is based on and claims the benefit of priority fromJapanese Patent Application No. 2021-042211, filed on 16 Mar. 2021, thecontent of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for processing an aluminumalloy and an aluminum alloy workpiece.

Related Art

As a processing method of a low silicon aluminum alloy, for example, amethod in which a low silicon aluminum alloy is subjected to casting,followed by hot forging is known.

As a method for obtaining a part. Patent Document 1 discloses a methodincluding: casting an alloy in a mold; after the casting, demolding thepart constituting a preform that is still hot; cooling the preform andthen subjecting it to an operation suitable for reheating it to atemperature range of from 470° C. to 550° C.; positioning the partbetween two shells of a die that defines a cavity of dimensionssubstantially equal to but less than the dimensions of the cavity of themold; and strongly pressing the two shells together to exert on the partdisposed between said shells a combined effect of pressing and surfacekneading. Herein, the low silicon aluminum alloy contains silicon at acontent between 0.5% and 3%, magnesium at a content between 0.65% and1%, copper at a content between 0.20% and 0.40%, manganese at a contentbetween 0.15% and 0.25%, titanium at a content between 0.10% and 0.20%,and strontium at a content between 0 ppm and 120 ppm.

Patent Document 1: Japanese Unexamined Patent Application (Translationof PCT Application), Publication No. 2018-507324

SUMMARY OF THE INVENTION

However, there has been a problem that an area average crystal grainsize of an aluminum alloy workpiece increases to about 800 μm, andvariation in the area average crystal grain sizes between sitesincreases.

An object of the present invention is to provide a method for processingan aluminum alloy, the method being able to reduce an area averagecrystal grain size and a variation in the area average crystal grainsizes between sites, and to provide an aluminum alloy workpiece.

An aspect of the present invention relates to a method for processing analuminum alloy containing: 0.5% by mass or more and 1.0% by mass or lessof Mg, 0.5% by mass or more and 3.0% by mass or less of Si, 0.2% by massor more and 0.4% by mass or less of Cu, 0.15% by mass or more and 0.25%by mass or less of Mn, 0.1% by mass or more and 0.2% by mass or less ofTi, and 120 ppm by mass or less of Sr, and the method includes castingthe aluminum alloy and forging the cast aluminum at a temperature of200° C. or more and 470° C. or less.

The cast aluminum alloy may be forged at a temperature of 400° C. ormore and 450° C. or less.

Another aspect of the present invention relates to an aluminum alloyworkpiece containing: 0.5% by mass or more and 1.0% by mass or less ofMg, 0.5% by mass or more and 3.0% by mass or less of Si, 0.2% by mass ormore and 0.4% by mass or less of Cu, 0.15% by mass or more and 0.25% bymass or less of Mn, 0.1% by mass or more and 0.2% by mass or less of Ti,and 120 ppm by mass or less of Sr, and having a Z parameter of 1.44×10⁹s⁻¹ or more and 1.18×10¹⁵ s⁻¹ or less.

The aluminum alloy workpiece may have a Z parameter of 2.73×10⁹ s⁻¹ ormore and 1.58×10¹⁰ s⁻¹ or less.

According to the present invention, it is possible to provide aprocessing method which is capable of reducing an area average crystalgrain size and a variation in area average crystal grain sizes betweensites, and to provide an aluminum alloy workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of the aluminum alloy workpiece of Example 1;

FIG. 2 presents diagrams of crystal orientation maps of three testpieces from each of the aluminum alloy workpieces of Examples 1 and 2and Comparative Example 1;

FIG. 3 is a diagram showing relationships between Z parameters and areaaverage crystal grain sizes of the three test pieces from each of thealuminum alloy workpieces of Examples 1 and 2 and Comparative Example 1;and

FIG. 4 is a diagram showing a relationship between forging temperaturesand variations in the area average crystal grain sizes, with respect tothe three test pieces from each of the aluminum alloy workpieces ofExamples 1 and 2 and Comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION Processing Method of AluminumAlloy

The processing method of an aluminum alloy of the present embodiment isa method of processing an aluminum alloy containing 0.5% by mass or moreand 1.0% by mass or less of Mg, 0.5% by mass or more and 3.0% by mass orless of Si, 0.2% by mass or more and 0.4% by mass or less of Cu, 0.15%by mass or more and 0.25% by mass or less of Mn, 0.1% by mass or moreand 0.2% by mass or less of Ti, and 120 ppm by mass or less of Sr.

The processing method of the aluminum alloy of the present embodimentIncludes casting an aluminum alloy, and forging the cast aluminum alloyat a temperature of 200° C. or more and 470° C. or less.

In the processing method of the aluminum alloy of the presentembodiment, an aluminum alloy having a composition as described above isforged at a temperature of 200° C. or more and 470° C. or less, wherebythe area average crystal grain sire and variation of area averagecrystal grain sizes between sites can be reduced. As a result, thealuminum alloy workpiece can be expected to have uniform elongationcharacteristics, and improved general corrosion resistance and stresscorrosion cracking resistance.

The forging temperature of the aluminum alloy is 200° C. or more and470° C. or less, and is preferably 400° C. or more and 450° C. or less.When the forging temperature of the aluminum alloy is less than 200° C.,hot forging of the aluminum alloy is not possible, and when the forgingtemperature exceeds 470° C., the aluminum alloy workpiece has a largerarea average crystal grain size and a larger variation in the areaaverage crystal grain sizes between sites.

The content of Mg in the aluminum alloy is 0.5% by mass or more and 1.0%by mass or less, and is preferably 0.5% by mass or more and 0.8% by massor less.

The content of Si in the aluminum alloy is 0.5% by mass or more and 3.0%by mass or less, and is preferably 1.5% by mass or more and 2.5% by massor less.

The content of Cu in the aluminum alloy is 0.2% by mass or more and 0.4%by mass or less, and is preferably 0.2% by mass or more and 0.3% by massor less.

The content of Mn in the aluminum alloy is 0.15% by mass or more and0.25% by mass or less, and is preferably 0.15% by mass or more and 0.2%by mass or less.

The content of Ti in the aluminum alloy is 0.1% by mass or more and 0.2%by mass or less, and is preferably 0.15% by mass or more and 0.2% bymass or less.

The content of Sr in the aluminum alloy is 120 ppm by mass or less, andis preferably 1 ppm by mass or less.

In addition to the above elements, the aluminum alloy may furthercontain B or the like.

The method of casting the aluminum alloy is not particularly limited,and examples thereof include gravity die casting (GDC), low pressure diecasting (LPDC), and the like.

When casting the aluminum alloy, the temperature of a holding furnacewhich holds molten metal in which the aluminum alloy is molten, is, forexample, 700° C. or more and 750° C. or less.

Further, when casting the aluminum alloy, the temperature of the moldis, for example, 150° C. or more and 200° C. or less.

When forging an aluminum alloy, the aluminum alloy is heated by using,for example, an electric furnace or the like.

When forging an aluminum alloy, a mold may be used. At this time, thetemperature of the mold is, for example, 150° C. or more and 200° C. orless.

The processing method of the aluminum alloy of the present embodimentmay further include a step of melting the forged aluminum alloy, and astep of artificially aging the aluminum alloy subjected to the meltingtreatment.

Conditions for melting the aluminum alloy are, for example, 5.5 hours ormore and 6 hours or less at a temperature of 530° C. or more and 540° C.or less. Further, conditions for the artificial aging treatment of thealuminum alloy are, for example, 4 hours or more and 7 hours or less ata temperature of 155° C. or more and 165° C. or less.

Aluminum Alloy Workpiece

The aluminum alloy workpiece of the present: embodiment is an aluminumalloy workpiece described above, having a Z parameter of 1.44×10⁹ s⁻¹ ormore and 1.18×10¹⁵ s⁻¹ or less. As a result, the aluminum alloyworkpiece of the present embodiment has a small area average crystalgrain size and a small variation in the area average crystal grain sizesbetween sites.

Here, the Z parameter can be obtained by an equation of Zener-Hollomon:

Z=A·ε·exp(Q/RT),

in which A is a material constant, ε is a strain rate, Q is anactivation energy, R is the gas constant, and T is an absolutetemperature. At this time, ε and T are a strain rate and an absolutetemperature, respectively, when forging the aluminum alloy.

When determining the Z parameter of the aluminum alloy workpiece of thepresent embodiment, A, Q, and R are set to 1, 142,000 J/mol, and 8.314J/mol K, respectively.

The Z parameter of the aluminum alloy workpiece of the presentembodiment is 1.44×10⁹ s⁻¹ or more and 1.18×10¹⁵ s⁻¹ or less, and ispreferably 2.73×10⁹ s⁻¹ or more and 1.58×10¹⁰ s⁻¹ or less.

The area average crystal grain size of the aluminum alloy workpiece ofthe present embodiment is preferably 300 μm or less.

The area average crystal grain size of the aluminum alloy workpiece ofthe present embodiment is usually 150 μm or more.

EXAMPLES

Hereinafter, the Examples of the present invention will be described,but the present invention is not limited to the Examples.

Example 1 Melting

An aluminum alloy ingot consisting of Mg (0.6% by mass). Si (1.8% bymass), Cu (0.2% by mass), Mn (0.15% by mass), Ti (0.17% by mass), Sr (1ppm by mass or less), and Al (balance) was melted using a meltingfurnace, to obtain a molten metal. At this time, the quality of thealuminum alloy ingot was measured using an inclusion analyzer, PoDFA(manufactured by Pyrotek Co., Ltd.), and it was confirmed that theimpurity amount was 0.2 mm²/kg or less. Furthermore, because aneffective addition amount of Mg varies with holding time in the meltingfurnace, deviation from a component target value was confirmed, usingoptical emission spectroscopy, and a Mg mother alloy was added to themolten metal to carry out component adjustment before casting.Furthermore, to improve the quality of the molten metal, degassing andfluxing with N₂ gas were performed.

Casting

The molten metal was conveyed into a holding furnace at 700° C., waspoured into a mold in a state of being heated to 200° C., and was castby GDC to obtain an intermediate. At this time, casting was performed soas to realize directional solidification by cooling the mold with wateruntil solidification of the molten metal was completed. Furthermore,burrs generated during casting were removed using a trimming device, toobtain an intermediate.

Forging

The intermediate was heated using an electric furnace until it reached400° C. (forging temperature). At this time, after confirming with athermocouple that the temperature of the surface of the intermediatereached 400° C., heating was continued for about 30 minutes so that auniform temperature would be obtained even in the inner part of theintermediate. Next, after confirming that the temperature of the moldreached 200° C., the intermediate was taken out from the electricfurnace, and the intermediate was forged using a forging machine.

Heat Treatment

The intermediate after forging was subjected to a melting treatment andan artificial aging treatment to obtain an aluminum alloy workpiece. Theconditions in the melting treatment were 6 hours at 540° C., and theconditions in the artificial aging treatment were 6.5 hours at 160° C.

Example 2

The same procedures were performed as in Example 1 to obtain an aluminumalloy workpiece, except that the forging temperature was changed to 470°C.

Comparative Example 1

The same procedures were performed as in Example 1, to obtain analuminum alloy workpiece, except that the forging temperature waschanged to 525° C.

FIG. 1 is a photograph showing the aluminum alloy workpiece ofExample 1. Incidentally, A, B and C in FIG. 1 indicate sites from whichthe test pieces were cut out when measuring the area crystal grain sizesof aluminum alloy workpieces to be described below.

Crystal Grain Sizes of Aluminum Alloy Workpieces

Three test pieces were cut. out from sites A, B, and C of each of thealuminum alloy workpieces (see FIG. 1). Next, the test pieces werepolished to about #2000 of polishing paper, and then were subjected tofinal polishing using colloidal silica and ion milling. Then, each ofthe test pieces was set in a scanning electron microscope (SEM), and anarea average crystal grain size of the test piece was measured usingelectron backscatter diffraction (EBSD). At this time, the grain sizeand area were acquired by setting a crystal misorientation of 15° ormore as a crystal grain boundary.

Here, if a simple average crystal grain size is used, difference betweenan apparent crystal grain size and the average crystal grain size isincreased, in a case in which a large number of crystal grains, eachhaving a small area, are contained in a structure in which variationexists in the crystal grain sizes. Therefore, an area average crystalgrain size d_(ave) was calculated using the following formula:

d _(ave)=Σ_(i)d_(i) A _(i)/Σ_(i) A _(i)   [Equation 1]

in which d_(i) is an elliptically approximated grain size of the i^(th)grain and A_(i) is an area of the i^(th) grain. Furthermore, differencebetween the maximum value and the minimum value of the area averagecrystal grain sizes of the three test pieces was obtained, and thedifference was used as the variation of the area average crystal, grainsizes.

FIG. 2 indicates crystal orientation maps of the three test pieces fromeach of the aluminum alloy workpieces of Examples 1 and 2 andComparative Example 1.

Table 1 shows the area average crystal grain sizes of the three testpieces from each of the aluminum alloy workpieces of Examples 1 and 2and Comparative Example 1.

TABLE 1 Area average grain size (μm) Z parameter (s⁻¹) Site A B C A B CExample 1 133 120 124  1.58 × 10¹⁰  2.58 × 10¹⁰  1.98 × 10¹⁰ Example 2260 151 188 1.44 × 10⁹ 2.36 × 10⁹ 1.81 × 10⁹ Comparative Example 1 354775 390 2.96 × 10⁸ 4.83 × 10⁸ 3.71 × 10⁸

FIG. 3 indicates relationships between Z parameters and area averagecrystal grain sizes of the three test pieces from each of the aluminumalloy workpieces of Examples 1 and 2 and Comparative Example 1.

From FIG. 3, it can be seen that the aluminum alloy workpieces ofExamples 1 and 2 had smaller area average crystal grain sizes than thealuminum alloy workpiece of Comparative Example 1.

FIG. 4 indicates a relationship between the forging temperatures and thevariations in the area average crystal grain sizes, with respect to thethree test pieces from each of the aluminum alloy workpieces of Examples1 and 2 and Comparative Example 1.

From FIG. 4, it can be seen that the aluminum alloy workpieces ofExamples 1 and 2 had smaller variations in the area average crystalgrain sizes than the aluminum alloy workpiece of Comparative Example 1.Furthermore, it can be seen from FIG. 4 that the variation in the areaaverage crystal grain sizes at 450° C. was about half the variation inthe area average crystal grain sizes at 470° C.

What is claimed is:
 1. A method for processing an aluminum alloycomprising: 0.5% by mass or more and 1.0% by mass or less of Mg, 0.5% bymass or more and 3.0% by mass or less of Si, 0.2% by mass or more and0.4% by mass or less of Cu, 0.15% by mass or more and 0.25% by mass orless of Mn, 0.1% by mass or more and 0.2% by mass or less of Ti, and 120ppm by mass or less of Sr, the method comprising casting the aluminumalloy and forging the cast aluminum at a temperature of 200° C. or moreand 470° C. or less.
 2. The method for processing the aluminum alloyaccording to claim 1, wherein the cast aluminum alloy is forged at atemperature of 400° C. or more and 450° C. or less.
 3. An aluminum alloyworkpiece, comprising: 0.5% by mass or more and 1.0% by mass or less ofMg, 0.5% by mass or more and 3.0% by mass or less of Si, 0.2% by mass ormore and 0.4% by mass or less of Cu, 0.15% by mass or more and 0.25% bymass or less of Mn, 0.1% by mass or more and 0.2% by mass or less of Ti,and 120 ppm by mass or less of Sr, and having a Z parameter of 1.44×10⁹s⁻¹ or more and 1.18×10¹⁵ s⁻¹ or less.
 4. The aluminum alloy workpieceaccording to claim 3, wherein the aluminum alloy workpiece has a Zparameter of 2.73×10⁹ s⁻¹ or more and 1.58×10¹⁰ s⁻¹ or less.